The problems associated with propellant pot life (which is defined hereinbelow) have increased in severity with the increased use of very fine ammonium perchlorate (less than 20 micrometers), and particularly, when ultra fine ammonium perchlorate (UFAP) (particle size generally from about 0.25 to about 5 micrometers) is used in the propellant formulation.
The use of burning rate catalysts, the use of energetic plasticizer compounds, and the use of small particle size oxidizer have been combinations employed to achieve the increased burning rates desired. These combinations attributed to decreased pot life. Pot life is the time during which the propellant mix retains its proper fluid properties to permit mixing and casting.
The definition of pot life teaches that without the proper time and fluid properties sufficient to permit mixing and casting, a propellant batch would be unusable.
Some viscosity control has been effected by use of lecithin; however, lecithin is a processing aid which is effective for effecting smaller changes in fluidity, providing aid in obtaining homogeneity of the propellant mix, and providing some improvement to pot life.
The use of coated ultrafine ammonium perchlorate offers an improvement which effects pot life. For example, U.S. Pat. No. 3,953,257, titled: "Method for Preparing Small Particle Size Coated Ammonium Perchlorate" and assigned to USA as represented by the Secretary of The Army, Washington, D.C., teaches a method for preparing aziridine coated ammonium perchlorate (AP) of weight median diameters (WMD) ranging from about 0.25 to about 5.0 micrometers. The coated AP product extends the pot life for hydroxy terminated polybutadiene (HTPB) propellant formulations.
U.S. Pat. No. 4,019,933, titled: "Pot Life Extension of Isocyanate Cured Propellants by Aziridine Compounds" and assigned to USA as represented by the Secretary of The Army, Washington, D.C., discloses and teaches the use of the reaction product formed from tri-aziridinylphosphine oxides or derivatives thereof reacted with monofunctional carboxylic acids. The reaction product is used to coat AP of less than 20 micrometers particle size which when employed in a propellant composition extend the pot life of the composition.
Shortened pot life has been attributed to the presence of ferric ions that have been released from ferrocene burning rate catalysts and derivatives of ferrocene. The iron content of ferrocene compounds such as n-butyl ferrocene, and the ferrocene derivatives sold as Hycat 6 and Catocene have made them attractive as burning rate catalysts. The ferrocene type catalysts have been tailored to offer various improvements to the propellant system. This tailoring accounts for the changes which are intended to overcome hazards associated with mixing ammonium perchlorate and volatile ferrocene compound, the tendency to migrate in a propellant composition, and the tendency to oxidize. Although certain undesirable features are overcome by the modified compounds, other features may develop or result from interactions between the propellant ingredients which produces other undesirable features. The change in pot life is an undesirable feature which has to be overcome, otherwise, the valuable benefits obtained by the tailoring cannot be put into use if the propellant cannot be processed properly.
An object of this invention is to provide an additive for solid propellant compositions for overcoming the shortened pot life resulting from employing certain ferrocene derivatives as the burning rate catalysts.
Another object of this invention is to provide an additive for solid propellant compositions which functions as a cure-rate inhibitor for ferrocene-containing propellants thereby extending the pot life for these propellants without inhibiting the final cure thereof.